The present invention relates to MxN fiber optic couplers, wherein M is equal to or greater than 1 and N is equal to or greater than 2, and to a method of making such couplers. The method of this invention is especially useful for making 1.times.2 and 2.times.2 couplers.
Fiber optic couplers referred to as "fused fiber couplers" have been formed by positioning a plurality of fibers in a side-by-side relationship along a suitable length thereof and fusing and stretching the fibers to reduce the spacings between the cores. Various coupler properties can be improved by encapsulating the coupling region of the fibers in a matrix glass to form an "overclad coupler". Portions of the fibers to be fused are inserted into the longitudinal bore of a glass tube having a refractive index lower than that of the fiber claddings. Each end of the longitudinal bore is typically connected to its respective tube end surface by a funnel. The tube midregion is collapsed onto the fibers; the central portion of the midregion is then drawn down to that diameter and coupling length which is necessary to obtain the desired coupling.
An end region of the resultant coupler is shown in FIG. 6. The midregion of tube 60 is illustrated as being collapsed on fibers 62 and 63. Protective coatings 64 and 65 extend into funnel 66. After the coupler has cooled, a drop of glue is applied to each funnel, and thereafter cured, to increase the pull strength of the fibers. To apply the glue, a hollow filament 68 that is connected to a vacuum source is inserted into funnel 66. The applicator needle of a glue-containing syringe (not shown) applies a drop of glue at the end of funnel 66. A sufficient amount of glue 67 is dispensed to cover the bare regions of the fibers that extend into the funnel. The drop of glue is drawn to the bottom of the funnel by the evacuated filament. A sufficient period of time can be allowed to elapse to permit the glue to "wick" or flow by capillary action between the fibers and the adjacent wall of the bore to form a bead that extends into the bore. The glue is then cured.
Various disadvantages can arise when the above-described method is employed. The glue in the funnel can shrink due to temperature change and become detached from one side of the funnel. This can cause a fiber located in the glue adjacent the point of detachment to bend and thus become stressed and subject to fatigue; such a fiber may also exhibit microbending loss. This finding has been reinforced by computer modeling of overclad fiber optic couplers. Modeling results on coupler designs with funnels have shown that encapsulating the epoxy around the full 360 degrees of the funnel interior tends to lead to significant axial strains (and stresses) of the fibers. Also, models of void areas and delaminations within the bore have shown the existence of large stresses on the fibers.
Elimination of the funnel could lead to a simpler process for making certain kinds of couplers. The glue could be more easily applied since the process would be less dependent upon the flow characteristics of the glue. This would permit the use of different glue application techniques and would allow a greater freedom in the choice of glues, for example, higher viscosity glues might be employed.